1. Field of the Invention
The present invention relates to optical communication, and more specifically, it relates to tunable add/drop filters used in optical communication.
2. Description of Related Art
In optical networks, a variety of methodologies have been provided in the prior art for optical switching and interconnecting of the transport network layers.
Cheung, (xe2x80x9cAcousto-Optic Tunable Filters in Narrowband WDM networks: System Issues and Network Applications,xe2x80x9d IEEE J. Sele. Area Comm. 8(6), 1015, 1990.) uses four 1xc3x97N demultiplexers and N""s 2xc3x972 optical switches. The structure is complicated and the interconnections are difficult.
An add/drop filter has been proposed by Glance at ATandT. (Glance, xe2x80x9cTunable add/drop optical filter providing arbitrary channel arrangementxe2x80x9d, IEEE Photon. Lett, 7(11), 1303, 1995 and U.S. Pat. No. 5,488,500.) This filter seeks to provide the advantage of arbitrary channel arrangement, but still suffers a costly 6 dB optical coupling loss, because of the two-array waveguide grating demultiplexers used in the structure.
Another type of wavelength-space switch (Dono et al, xe2x80x9cA wavelength division multiple access network for computer communicationxe2x80x9d, IEEE J. Sol. Area Comm., 8(6), 983, 1990.) has been widely used in various WDM networks, for example the IBM Rainbow Network. This structure uses a passive star-coupler that combines and splits the incoming light signals into N receivers. The receivers are built with a tunable filter and select the desired channels. It has the broadcast capability and the control structure of this implementation is very simple. However, an undesirable feature of the broadcast star is that the splitting loss can be very high when the users number is large.
U.S. Pat. No. RE037,044, titled xe2x80x9cTunable Add/Drop Optical Filterxe2x80x9d describes a tunable optical add/drop filter for all-optical wavelength-division-multiplexing (WDM) network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It is intended for use to decentralized access points in the access network or as a small core network node to realize branching points in the network topology. It is intended to work in both the wavelength and space domains.
It is an object of the present invention to provide embodiments of a wide band tunable filter.
It is another object of the invention to provide techniques for maintaining the beam propagation directions over the wavelength tuning range of a tunable filter.
It is another object of the invention to provide techniques for maintaining the beam separation over the wavelength tuning range of a tunable filter.
Still another object is to provide a four-port device that can add and drop wavelength channels simultaneously.
Another object of the invention is to provide a thermal compensator to compensate for heat induced pass-band changes.
These and other object will be apparent to those skilled in the art based on the disclosure herein.
According to the present invention, in one embodiment of a tunable filter, a randomly polarized incoming beam normally incident upon a birefringent crystal is separated into separate o- and e-ray beams. The E-field (polarization) orientation of the o-ray, after emerging from the crystal, is orthogonal to that of the e-ray. A xc2xd wave plate placed in one beam causes the two rays to have the same polarization direction. A filter block comprising optically transparent material, and further comprising a mirror and a bandpass filter is placed in the path of the o-ray and the e-ray. Wavelengths within the passband of the filter are passed for the o- and e-rays. The wavelengths not within the passband are reflected from the filter and reflected by the mirror to produce second o-ray and e-ray beams that include all of the light that was not passed by the passband filter. Thus, the filter block turns the o-ray into a top o-ray and a bottom o-ray and turns the e-ray into a top e-ray and a bottom e-ray. In this embodiment, the bottom rays carry light of wavelengths that are within the filter""s pass band and the top rays carry the rest of light.
A birefringent crystal is positioned after the filter block. A xc2xd wave plate is attached to the crystal to intercept the top e-ray and bottom e-ray, but not to intercept the top o-ray and the bottom o-ray, The xc2xd wave plate rotates the polarization direction of top e-ray and bottom e-ray to be orthogonal to the polarizations of the top o-ray and the bottom o-ray. The top o-ray and top e-ray combine in the second birefringent crystal to produce a combined top beam. The bottom o-ray and bottom e-ray combine in the crystal to form a combined bottom beam.
An embodiment of the invention is thus a three-port device. From one input beam, the invention produces two output beams. The input port carries all the wavelengths injected into the system and the bottom output port carries the light with wavelengths that pass through the filter. The top output port carries the light reflected by the filter.
Embodiments of the invention include configurations where light reflected by the mirror impinges on a second drop filter, which passes certain wavelengths and reflects all the others. The mirror can be formed from a coating over the incidence surface of filter block, which surface has a non-coated portion to allow entry of the o and e rays. The exit surface of the filter block can have a plurality of bandpass filters positioned to successively pass selected wavelengths. In an alternate embodiment, a series of the devices of FIGS. 1A and 1B are connected to the top output port to operate as a demultiplexer. These principles apply to the embodiments provided below as well.
The wavelength in the bottom output port is determined by the transmission band of the filter, which can be adjusted by changing the incident angle to the filter. The disadvantage is that, as the incident angle changes, the two output beams shift laterally. This problem is fixed by adding a plane parallel plate (dummy block) in the optical path. The material and thickness of dummy block are the same as those of the filter block. Adding another plane parallel plate to the bottom beam path compensates for the thickness of filter. When the filter block is rotated, the dummy block is correspondingly rotated in the opposite direction. Such arrangement will guarantee that the bottom beam remains in the same location no matter how the filter block is rotated.
The invention includes embodiments that substitute a first and second polarizing beamsplitter (PBS) and mirror combination for the birefringent crystals of the above-described embodiment.
Another embodiment is provided which places a bandpass filter coating on the input side of the filter block. Wavelengths within the band pass of the filter coating are transmitted through the filter and the remaining wavelengths are reflected to a corner cube. The corner cube reflects light incident thereon back to a reflective coating placed on the input side of the filter block. The light reflected from this mirror coating is reflected back towards the input direction. This embodiment is a three-port device.
A 4-port device is provided, which can add and drop wavelengths simultaneously. In this embodiment, the filter block includes a filter coating, an attached mirror and an attached mirror block. The mirror block has an attached mirror. A dummy block with an adherent dummy glass is operatively positioned next to mirror block. In operation, an input beam passes through the filter block and impinges on filter coating. Light having wavelengths within the passband of filter coating will pass through the filter coating and will pass through the dummy glass. The remaining light will be reflected by the filter coating and be further reflected by the mirror, from which the light will propagate through the dummy block. In one embodiment, the filter coating is designed to reflect a narrow wavelength band, such that a single wavelength is reflected therefrom. The reflected channel becomes the main channel of the device. An added channel is introduced into the system by injecting a beam through filter block and mirror block so that the beam reflects from the mirror attached to the mirror block and is made collinear with the other beam reflected from the filter and propagates therewith out of the system.
The separation between e- and o-ray are not necessary when the incident angle is close to normal incident Since under that condition, the filter response to P- and S-polarization is about the same. The disadvantage of operating in the small incident angle is that the tuning range is small. The advantage is that one does not need to convert the incident polarization into a pure S- or P-state.
Assuming that when the incident angle onto the filter is fixed, higher temperatures shift the filter pass-band to the longer side. The invention includes a thermal compensator to correct the angle of the filter with respect to the incident light